1. Field of the Invention
The present invention relates to a method of forming a supported bore, and in particular, but not exclusively, to a method of forming a subterranean well bore. The present invention also relates to an apparatus for forming a supported bore.
2. Description of the Related Art
In the oil and gas exploration and extraction industry, hydrocarbons contained within subterranean formations are accessed and flowed to surface via well bores drilled from surface to intersect the formations. Conventional methods of producing a well bore involve a staged process. Initially, a bore section is drilled using a drill bit mounted on a drill string comprising a combination of drilling pipes and collars, which are used to impart rotation and weight to the drill bit. After a certain depth, however, the walls of the bore are likely to become unstable and will eventually collapse if unsupported. Thus, to prevent collapse, the next stage involves pulling the drill bit and the drill string out of the hole to allow a supporting structure to be set in place. Often a bore will be drilled to a depth of between 610 to 915 m (2000 to 3000 ft) before support is provided. Thus, pulling the drill string out of the hole is extremely time consuming, considering that conventional drilling rigs and platforms can accommodate dismantling and storage of the drill string in lengths, known as “stands”, of approximately 27.5 m (90 ft).
As noted above, once the drill bit and drill string have been removed from the hole a supporting structure is set in place. The supporting structure, generally referred to as the casing or liner, comprises a number of casing tubulars or liner tubulars which are normally coupled together by threaded connectors and extend to the bottom of the hole. Once the casing or liner is run into the bore, a cement slurry is pumped into the annulus formed between the wall of the bore and the casing, which cement cures to set and seal the casing in place.
Once the supporting structure is set in place, a slightly smaller diameter drill bit may then be run in the newly cased hole on the drill string, which has to be reassembled. Drilling may re-commence once the previous depth is achieved by the drill bit.
The above procedure is then repeated as necessary until the total required depth of the bore is accomplished, and as will be appreciated, the time taken to pull the drill bit and drill string out of the hole, and to run new casing or liner into the hole will increase after each drilling operation due to the increasing depth of the bore.
Furthermore, as the bore is progressively drilled in the manner described above, various readings must be taken in the bore before the casing is cemented in place, such as depth, temperature, pressure, formation characteristics and the lice. Acquiring such readings is generally termed “logging” or “open hole logging” and is normally carried out with the drill bit removed from the unsupported hole. Some logging operations, however, may be achieved with the drill bit located in the hole, and even while drilling, by utilizing Logging While Drilling (LWD) tools or Measurement While Drilling (MWD) tools, which can be elaborate and expensive.